Two closely related species of hamsters--Syrian and Turkish--will be studied with respect to factors involved in hibernation. Hamsters will be exposed to decreased photoperiods prior to being exposed to cold to determine whether daylength serves as a cue to prepare these animals for hibernation. Possible effects of gonadal and pineal hormones as inhibitors or facilitators of hibernation will also be examined. Growth and histology of the testis will be studied in hibernating hamsters, as will the responsiveness of the testes to pituitary gonadotropins. The role of thyroid and adrenal hormones in the regulation of metabolism in torpid and arousing hamsters will be investigated by employing drugs which block the synthesis of these hormones.